On-sun test results from second-generation and advanced-concepts alkali-metal pool-boiler receivers

ABSTRACT:
Two 75-kWt alkali-metal pool-boiler solar receivers have been successfully tested at Sandia National Laboratories' National Solar Thermal Test Facility. The first one, Sandia’s “second generation pool-boiler receiver”, was designed to address commercialization issues identified

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during post-test assessment of Sandia’s first-generation pool-boiler receiver. It was constructed from Haynes alloy 230 and contained the alkali-metal alloy NaK-78. The absorber’s wetted side had a brazed-on powder-metal coating to stabilize boiling. This receiver was evaluated for boiling stability., hot- and warm-restart behavior, and thermal efficiency. Boiling was stable under all conditions. All of the hot restarts were successful. Mild transient hot spots observed during some hot restarts were eliminated by the addition of 1/3 torr of xenon to the vapor space. All of the warm restarts were also successful. The heat-transfer crisis that damaged the first receiver did not recur. Thermal efficiency was 92.3% at 750°C with 69.6 kWt solar input. The second receiver tested, Sandia’s “advanced-concepts receiver,” was a replica of the first generation receiver except that the cavities, which were electric-discharge-machined in the absorber for boiling stability, were eliminated. This step was motivated by bench-scale test results that showed that boiling stability improved with increased heated-surface area, tilt of the heated surface from vertical, and added xenon. The bench-scale results suggested that stable boiling might be possible without heated-surface modification in a 75-kWt receiver. Boiling in the advanced concepts receiver with 1/3 torr of xenon added has been stable under all conditions, confining the bench-scale tests.

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